Magnetic properties of Co/Ni multilayer structures for use in STT-RAM

Magnetic properties of (111)-textured SAF/Cu/FL multilayer film structures have been optimized by varying individual layer thickness and sputtering conditions. The SAF is a synthetic antiferromagnet consisting of Co/Ni multilayers coupled antiferromagnetically across a Ru spacer layer, and the FL is a free layer consisting of a single Co/Ni multilayer. The Co thickness was varied from 0.16 nm to 0.26 nm and the Ni thickness from 0.47 nm to 0.57 nm at two different deposition rates to obtain a perpendicular magnetic anisotropy of the 8 × [Co/Ni] multilayers ranging from 4×10⁵ to 4.6×10⁵ Jm^-3. The perpendicular magnetic anisotropy, saturation magnetization, damping and zero-frequency line broadening of the Co/Ni multilayers strongly depend on the number of bilayers, N₁, for N₁ <10. With increasing Cu seed-layer thickness, the texture of the Co/Ni multilayers improves while the grain size and film roughness increase. The increase in grain size results in the reduction of the direct exchange coupling between magnetic grains, which enhances the coercivity of the SAF and the FL. Experimentally measured coercivities of the SAF and FL are compared with calculations obtained from a coherent rotation model. The effect of the Co/Ru interface in the SAF on the exchange coupling, and the role of the Co/Cu interface in the magnetoresistance, is also discussed. This study provides an important step towards the realization of STT-RAM devices consisting of only Co/Ni multilayers.